Heretofore, state-of-the-art optic modulators have used either polarization of light, such as in nematic liquid crystal devices, or absorption of light, such as in devices based on the Quantum Confined Stark Effect (QCSE), to modulate optic signals. These devices, however, resulted in either high contrast (10,000:1) and low speed (1 msec-1 usec), liquid crystal devices, or high speed (100 psec-10 msec) and low contrast (100:1), QCSE devices.
As is well known, nematic liquid crystal devices are generally composed of two glass plates with conductive coatings on their inner surfaces and liquid crystal being sandwiched between them. When subjected to an electric field by the conductive coatings, the liquid crystal becomes opaque and reflective thus modulating any light incident to the device.
QCSE devices, in contrast to liquid crystal devices, utilize quantum well superlattice heterostructures and the application of an electric field to vary the absorption resonance of the heterostructure which, in turn, modulates optic signals. The principles behind the QCSE have been more fully explained by D. A. B. Miller et al, in Physics Review, 1985, B32, pg. 1043. Briefly though, in a quantum well at zero electric field, the electron and hole energy levels of the heterostructure are defined by the well width, and generally it is preferred that the electrons and holes are strongly confined in the well layer. When an electric field is applied, the electrons and holes are moved apart and their energies altered. This has the effect of shifting the absorption resonance to lower energy as well as modulating the strength of the absorption. This occurs because direct optical absorption of a photon above the band gap energy involves raising an electron from one of the valence bands and putting it in the conduction band. This is otherwise known as formation of an electron-hole pair. This shift in the absorption resonance, then, provides for the optical modulation of any radiation that is incident to the heterostructure.
As indicated above, the devices based on the QCSE have high speed and poor contrast while liquid crystal devices have low speed and high contrast. Therefore, there still remains a need for a high contrast and high speed optic modulator. The present invention fulfills such a need.